1. Field of the Invention
The present invention relates to a can type lithium secondary battery. More particularly, the present invention relates can type lithium secondary battery configured to short-circuit in a predetermined manner when subjected to an externally-applied force, thereby providing an enhanced margin of safety.
2. Description of the Prior Art
Batteries are employed in a wide variety of equipment including, to name just a few, vehicles, e.g., electric and hybrid vehicles, portable power tools, electronics, etc. Portable compact electronic devices, e.g., cellular phones, laptop computers, camcorders, etc., are being widely developed and produced. Such portable electronic devices typically include an internal battery pack to allow continuing operation even when no external power supply is available. Such a built-in battery pack typically includes at least one unit battery inside and provides the electronic device with a predetermined level of voltage over a reasonable period of time.
Batteries are generally classified as primary or secondary batteries. Primary batteries are commonly known as single-use batteries, while secondary batteries are commonly known as rechargeable batteries. Both primary and secondary batteries may be suitable for use in portable electronic devices. However, secondary batteries are widely adopted because they can be reused and may be, therefore, more economical than single-use batteries. Secondary batteries have been developed using a wide variety of technologies.
Battery technologies currently favored for portable electronic devices include nickel cadmium (Ni—Cd) batteries, nickel metal hydride (Ni-MH) batteries, lithium (Li) batteries, etc. The Li batteries have, in particular, been widely employed in the latest generation of electronics devices. A Li battery may have an operation voltage of 3.6 V, which is about three times the operation voltage of the comparable Ni—Cd or Ni-MH batteries. The Li battery may exhibit a relatively high energy density per unit weight. In a typical Li battery, a lithium-based oxide may be used as a positive electrode activation material, and a carbon-based material may be used as a negative electrode activation material. Li batteries may be classified as liquid electrolyte batteries and polymer electrolyte batteries, depending on the electrolyte used therein. Liquid electrolyte batteries are also known as lithium ion (Li-ion) batteries and polymer electrolyte batteries are also known as Li polymer batteries. The Li battery may be manufactured in various shapes, e.g., cylindrical can types, rectangular or prismatic can types, pouch types, etc.
A typical can type lithium secondary battery may have an electrode assembly, a battery case for housing the electrode assembly and electrolyte injected inside the battery case to give mobility to charge-carrying ions. The electrode assembly may include a positive electrode plate on which a positive activation material is coated, a negative electrode plate on which a negative activation material is coated and a separator interposed between the positive and negative electrode plates. The separator may serve to prevent short circuits between the positive and negative electrode plates, and to allow only ions to pass. The width of the separator of the electrode assembly may be larger than the widths of the electrode plates, in order to prevent the electrode plates from making contact with each other. However, if such a battery suffers an external impact, e.g., by being dropped or hit, it may develop a short circuit.
FIG. 1 illustrates an exploded perspective view of a conventional can type lithium secondary battery. Referring to FIG. 1, the can type lithium secondary battery may include an electrode assembly 112 including an anode electrode plate 113, a cathode electrode plate 115 and a separator 114. The can type lithium secondary battery may further include a can 110 for receiving the electrode assembly 112 and an electrolyte, and a cap assembly 120 for tightly sealing an upper opening 110a of the can 110.
The cap assembly 120 may include a cap plate 140, an insulation plate 150, a terminal plate 160 and an electrode terminal 130. The cap assembly 120 may be associated with a separate insulation case 170 and then coupled to the upper opening 110a of the can 110, so as to tightly seal the can 110.
The cap plate 140 may be a metal plate having a size and shape corresponding to those of the upper opening 110a of the can 110. The cap plate 140 may have a through-hole 141, which may be formed at a center portion thereof. The through-hole 141 may have a predetermined size for receiving the electrode terminal 130. A gasket 146, e.g., a gasket having a tubular aspect for encircling the electrode terminal 130, may be mounted on the electrode terminal 130 when the electrode terminal 130 is inserted into the through-hole 141, in order to insulate the electrode terminal 130 from the cap plate 140. The cap plate 140 may have an electrolyte injection hole 142, which may be formed at a side thereof. After the cap assembly 120 is coupled to the upper opening 110a of the can 110, the electrolyte may be injected through the electrolyte injection hole 142 into the can 110. Then, the electrolyte injection hole 142 may be sealed by separate sealing structure 143. The sealing structure 143 may be, e.g., a plug.
The electrode terminal 130 may be connected to a cathode electrode tap 117 of the cathode electrode plate 115, or to an anode electrode tap 116 of the anode electrode plate 113. The electrode terminal 130 may operate as a cathode electrode terminal or an anode electrode terminal.
The insulation plate 150 may be made from the same insulation material as the gasket 146 and may be attached to a lower surface of the cap plate 140. The insulation plate 150 may have a through-hole 151 formed in a portion thereof corresponding to the through-hole 141 of the cap plate 140, for receiving the electrode terminal 130. The insulation plate 150 may have a recess 152 formed in a lower surface thereof, in order to receive the terminal plate 160.
The terminal plate 160 may be made of, e.g., a metal such as nickel or nickel alloy, and may be attached to the lower surface of the insulation plate 150. The terminal plate 160 may have a through-hole 161 formed in a portion thereof corresponding to the through-hole 141 of the cap plate 140, for receiving the electrode terminal 130. The electrode terminal 130 may extend through the through-hole 141 and may be insulated from the cap plate 140 by the gasket 146, and the terminal plate 160 may be electrically insulated from the cap plate 140 while being electrically connected to the electrode terminal 130.
When the electrode terminal 130 is assembled with the cap plate 140, the insulation plate 150 and the terminal plate 160, the electrode terminal 130 may be inserted into the through-hole 141 by being rotated and subjected to a suitable force. After passing though the through-hole 141, the electrode terminal 130 may extend through the through-holes 151 and 161. The through-hole 151 may have a diameter equal to or slightly larger than a diameter of the electrode terminal 130. When the electrode terminal 130 is inserted into the through-hole 151, the peripheral surface of the electrode terminal 130 may come into close contact with and fit in the through-hole 151.
If a short circuit, e.g., an interior short circuit or exterior short circuit of the electrode assembly, occurs in the can type lithium secondary battery described above, an electric current flow may rapidly increase, which may result in the generation of a large temperature increase. Similarly, an overcharge or overdischarge of the can type lithium secondary battery may generate a large temperature increase. As a result, there is a danger of damage to the can type lithium secondary battery, the device the can type lithium secondary battery is installed in and/or the user.
In order to prevent interior short circuits of the can type lithium secondary battery, an insulating tape may be wound on various portions of the can type lithium secondary battery at which the short circuits are likely to occur, including, e.g., terminal portions of the anode and cathode electrode plates and the regions in which the electrode taps are welded in the electrode assembly. Additionally, safety elements, e.g., positive temperature coefficient devices, thermal fuses and protecting circuits, may be built into the can type lithium secondary battery. Such safety elements may interrupt the electric current before it exceeds a safe level, so as to prevent the damage of the can type lithium secondary battery.
However, if the can type lithium secondary battery described above is deformed by an external impact or externally-applied force, the safety elements may not prevent short circuiting between the electrode plates. In particular, when subjected to a standardized compression test or crush test used to evaluate safety for can type secondary batteries, the can type lithium secondary battery may suffer a short circuit between the electrode plates.
In the standardized compression test, a pressure jig is employed to apply a force to the can type lithium secondary battery. The pressure jig is used to compress two sides of the can type lithium secondary battery in a direction normal to the longitudinal axis of the can type lithium secondary battery. In particular, the pressure jig presses the can type lithium secondary battery, such that the pressing surfaces of the pressure jig are kept in parallel with the two sides of the can type lithium secondary battery, and subjects the can type lithium secondary battery to a pressure of about 13 kN in the direction normal to the longitudinal axis of the can type lithium secondary battery. Where the can type lithium secondary battery is a prismatic battery, the test may be repeated along a second axis, using a second sample of the can type lithium secondary battery. That is, referring to FIG. 1, the two short sides of a first sample of the can type lithium secondary battery may be crushed by applying the compressive force of a pressure of about 13 kN applied in the direction of the crush axis CA1.
When a can type lithium secondary battery is compressed according to the standardized compression test method, the anode electrode plate and the cathode electrode plate may be short circuited to each other, thereby allowing the electric current to rapidly flow from the anode electrode plate to the cathode electrode plate. This may result in a large amount of heat being generated by the resistance of the anode and cathode electrode plates. If the amount of heat generated is excessive, the can type lithium secondary battery may explode.